In optical fiber transmission systems, when the polarization multiplexing scheme that transmits signals using polarized lights with orthogonal polarization is applied, it becomes possible to double the transmission capacity per an optical fiber. In recent years, a digital signal processing technology is applied to a receiver of an optical transceiver, thereby enabling an efficient separation of a polarization multiplexed signal. Hence, the polarization multiplexing scheme is now widely used.
However, it is known that polarization multiplexed signals are affected by non-linear effects (intra-polarized-wave mutual phase modulation: intra-polarized-wave XPM) from orthogonally polarized signals. In addition, polarization multiplexed signals with the same wavelength not only propagate at the same speed in an optical fiber, but also cause similar waveform change when a wavelength dispersion occurs. Accordingly, an adverse effect of the intra-polarized-signal XPM is likely to be accumulated, and thus the signal quality is remarkably deteriorated in accordance with a transmission distance. Hence, in order to transmit the polarization multiplexed signals over a long distance while maintaining the excellent transmission quality, a technology of compensating or easing the waveform distortion caused during the transmission is important.
PTL 1 discloses that, in a transmitter of a light signal transmission system employing the polarization multiplexing scheme, by asymmetrical chirp is added to two light signals (polarized wave components) to perform phase modulation on the two light signals, those signals with polarized conditions orthogonal to each other are synthesized and a polarization multiplexed signal is generated.
According to the technology disclosed in PTL 1, when a wavelength dispersion is caused to the polarization multiplexed signal in a transmission line fiber, the one polarized wave component with an asymmetrical chirp has a time waveform dispersed (spread), while the other polarized wave component has a time waveform compressed (pulse compression). Hence, when the polarization multiplexed signal propagates in an optical fiber, a different waveform change (dispersion or compression) is shown between polarized waves orthogonal to each other at an arbitrary time slot. This is a waveform change that has a weak correlation between the polarized waves, and thus the adverse effect of intra-polarized-wave XPM can be eased.